If you have not synchronized your contacts during your registration, you can do so at any point in the application where you have the option to search for users (e.g. in Chat&Pay). By choosing to sync contacts, you will be redirected to your mobile settings, where you will be asked to provide the payzy app with access to your contacts.
Additionally, you can go to your profile by selecting "Mobile contacts on payzy" in order to synchronize your contacts.